It is obvious the subcritical multiplication factor significantly rises:

Because the **subcritical multiplication factor** is **related to** the value of k_{eff} (**core subcriticality**), it is possible to monitor the approach to criticality through the use of the subcritical multiplication factor. The closer the reactor is to criticality, the faster M will increase for equal step insertions of positive reactivity. When the reactor becomes critical, M will be infinitely large. Monitoring and **plotting M** during a criticality approach is impractical because there is no value of M at which the reactor clearly becomes critical.

On the other hand, the neutron flux is monitored via **source-range detectors**, where the **count rate (CR)** gets infinitely large as the core approaches k_{eff} = 1.0. Note that the count rate of source-range detectors is proportional to the neutron population (n ∝ CR). which can be determined by the source-range count rate.

Therefore, instead of plotting M directly, its inverse (1/M or 1/CR) is plotted on a graph of:

- 1/CR versus rod elevation (in case of criticality approach by control rod withdrawal)
- 1/CR versus boron concentration (in case of criticality approach by boron dilution)
- 1/CR versus the number of fuel assemblies (in case of loading of fuel into the core)

Note that a true 1/M plot requires knowledge of the neutron source strength. Because the actual source strength is usually unknown, a r**eference count rate** is substituted. As criticality is approached, 1/CR approaches zero. Therefore in startup procedure, the value of 1/CR provides engineers with an effective tool for monitoring the approach to criticality.

**Example of 1/M Plot**

Assume a criticality approach by controlling rod withdrawal. For this procedure, operators check especially the control rod’s position (usually in steps) and the count rate (CR) from source-range detectors. The initial count rate on the detectors prior to rod withdrawal is 100 cps. There are main parameters in the following table. From these parameters, the 1/M Plot is constructed. As can be seen, the estimated critical parameters can be predicted with extrapolation to the point where CR_{0}/CR → 0.